Building with a cljsjs package where cljsjs package includes :libs in deps.cljs you find the "out" directory includes a subdirectory "file:".
An example from my specific case:
':' is not permitted on Windows filesystem, which leads to compilation error.
It happens because rel-output-path here https://github.com/clojure/clojurescript/blob/17bcf2a091accb6f7caf1e8fa3954b490e9d34fa/src/main/clojure/cljs/closure.clj#L1515 detects closure-lib first (before taking in account that it is in jar) and passes it to lib-rel-path which acts as if file is located in project directory.
Another issue, but deeply related, is that on Windows, lib-rel-path breaks build even for :lib files located in project directory. It happens because of naive path prefix removal https://github.com/clojure/clojurescript/blob/17bcf2a091accb6f7caf1e8fa3954b490e9d34fa/src/main/clojure/cljs/closure.clj#L1500 because it tries to remove a subpath with native path separators, but path comes here in url-like form with forward slashes. Compiler then reports that path is not relative and aborts compilation.
Bug presents in any environment, but break things only on Windows.